Phase relations and diffusion layer formation in the systems Cu-Nb-Sn and Cu-Nb-Ge

Condensed phase relations were determined for part of the system Cu−Nb−Sn at 1100 and 1000°C, and for part of the system Cu−Nb−Ge at 1100°C. Diffusion experiments in both systems were conducted at 1100 and ∼800°C. The two types of experiments were compared in order to undertand the diffusion synthesis of superconducting A15 structure compounds from Cu−Ge or Cu−Sn bronzes. Whether an A15 layer of Nb₃Ge or Nb₃Sn can form by diffusion depends on the orientation of two-phase tielines in the appropriate system. The tielines existing between bronze and Nb₃Sn in the Cu−Nb−Sn system are closely followed by the diffusion path inbrozen-Nb diffusion couples so that a super conducting Nb₃Sn layer forms. Bronze-Nb₃Ge tielines are precluded in the Cu−Nb−Ge system by a two-phase field, Nb−Nb₅Ge(Cu) solid solution. Diffusion paths from bronze to Nb do not enter the Nb₃Ge phase field and no Nb₃Ge diffusion layer forms. The possibility exists that the addition of other chemical components might favorably modify the phase relations such that bronze diffusion synthesis can succeed for Nb₃Ge.